The following is provided as background information only and is not intended and is not to be construed as being prior art to the invention herein.
Electrochromism is a reversible, visible change in optical absorption in response to a dc voltage change. When certain chemical species are placed in an electrical field, the field induces a change in the reduction-oxidation (redox) state of the species. The change in redox state is accompanied by a change in visible color. Typically, the electrochromic species change from a high-transmittance, colorless or lightly colored state to a low transmittance, highly colored state. When the electrical field is reversed, these electrochromic species revert to their original color.
An electrochromic device (ECD) is comprised of at least two conducting or “electrode” layers, at least two electrochromic layers and one electrolyte layer dispersed between the electrochromic layers. The device, sometimes referred to as a “cell” in reference to its resemblance to an battery cell, can be a solution (in which case there are no actual layers), a thin-film or a hybrid, i.e., solid conducting and electrochromic layers with a liquid electrolyte layer. One of the first reports of an ECD was published in the late 1960s (S. K. Deb, Appl. Opt., 1969, Suppl. 3:193).
During the last three decades, ECDs have found a host of applications such as, without limitation, antiglare mirrors, secondary batteries, static displays and solar-attenuated windows. They have been especially useful in this last application, windows, where they serve as both light and heat regulating devices, since they are particularly amenable to relatively large surface areas.
Complementary electrochromic devices are those in which two different electrochromic chemical species that have an additive optical relationship to one another are used. That is, one of the species changes from transmissive to colored at a positive voltage, that is at the cathode side of the ECD and the other species changes from transmissive to colored at the corresponding negative voltage at the anode of the ECD. The value of complementary systems lies in their ability to achieve a greater transmittance window, i.e., the difference between the maximum bleached (colorless or lightly colored, i.e., transmissive) state of the device and the minimum darkened (highly colored) state, than single electrochromic species devices. Thus, one of the complementary species changes from a high transmittance, colorless or lightly colored state to a relatively low transmittance, colored state under the influence of a positive electric field and then reverts to its original state when the field is reversed and its complement reacts in exactly the opposite manner; that is, under a positive potential it is colorless or lightly colored and highly transmissive while under a negative potential it is colored and of relatively low transmittance. Examples of complementary electrochromic devices include poly(3,4-ethylenedioxythiophene) as the cathodically coloring species and certain metal oxides or conductive polymers as the anodic counter-electrode species (U.S. Pat. No. 6,157,479; S. A. Sapp, et al., Adv. Mater., 1996, 8:808-11; M. A. DePaoli, et al., Electrochim. Acta, 2001, 46:4243-49). Such systems are, however, problematic. With regard to metal oxide-based devices, the transmittance window, the difference between the maximum bleached state transmittance and the minimum darkened state transmittance is generally quite limited, less than 40% in most cases. On the other hand, while the transmittance window for conductive polymer counter-electrode devices is substantially greater than that of the metal oxide devices, their stability, that is, the number of times the device can be switched back and forth between a high transmittance and a low transmittance state while maintaining its transmittance window, is substantially lower.
What is needed is a complementary electrochromic device that provides both a large transmittance window together with long term dynamic (under application of switching electric fields) and static (resting state) stability. The present invention provides such complementary electrochromic devices.